The Migration of the Palaeotropical Arid Flora: Zygophylloideae as an Example

The rate and direction of biotic exchange between the Palaeotropical arid floras of Asia, Africa, and Australia is poorly understood because of a lack of phylogenetic hypotheses for relevant plant groups. Periodic aridification may have facilitated migrations of arid-adapted plants between southwestern Africa and the Horn of Africa as recently as the last glacial maximum, allowing further exchange with the arid floras of Asia. However, no conclusive evidence of the age and direction of such migrations have been documented. We use a molecular phylogeny of the Zygophylloideae to infer a biogeographic scenario for the arid Palaeotropics, using relaxed clock dating and likelihood and parsimony based ancestral area reconstruction methods. We infer up to five migrations across the African continent (in contrast to just one each to Australia and Asia from Africa). The three most recent were in the Pliocene/Pleistocene and from southern to northern Africa, while the oldest dates to the Oligoccene to Miocene. For the recruitment of the arid Palaeotropical flora, the preponderance of migrations across the African continent points to a repeated pattern of dispersal mediated by periodically more contiguous habitat, the so called ‘African arid corridor,’ with rarer long distance dispersal events between other disjunct areas.